Liquid spraying has been used to great advantage for many years in many applications. One of the most common uses for liquid spraying is for surface coating of substrates with thin layers of paint or varnish, for example. The present invention is directed to liquid spraying techniques and apparatus, but is not limited to any particular liquid.
Although there are many known techniques for spraying a liquid, each of these techniques can generally be characterized as an air spraying technique or an airless spraying technique. The present invention is directed to the latter techniques.
Both airless and air spray techniques are accomplished by forcing a liquid through a small orifice. According to the air spray technique, the liquid is intermixed with pressurized air and the mixture is sprayed through an orifice under pressure. Typically, the air pressure is on the order of approximately 10-50 pounds per square inch gauge (psi). In view of this, it is generally perceived that air spraying is reasonably safe because the driving air pressure is relatively low and therefore the velocity of the liquid droplets near the orifice is correspondingly low. However, air spraying requires not only a source of liquid (e.g., paint) but also requires access to a supply of pressurized air.
In airless spraying, the liquid itself it maintained under high hydraulic pressure, typically on the order of approximately 500-3,000 psi and this highly pressurized liquid is forced through a spray tip orifice to create the necessary atomization. Since an auxiliary air supply is unnecessary, and in view of superior functional characteristics and economies, airless spraying is perceived to possess several advantages over air spraying for some applications. However, the large hydraulic pressures contribute to a very high exit velocity near the downstream end of the spray tip orifice. In fact, the velocity near the spray tip can be so high that the exiting stream can, in some instances, penetrate human flesh when the flesh is positioned closely adjacent the spray tip orifice. When such penetration occurs, the fluid injected through the flesh may spread out along the underlying muscle layer, and, especially when the fluid is toxic as in the case of some paint ingredients, cause serious injury.
In general, there is a potential for injection of liquid dispensed from an airless spraying apparatus only if the flesh is very close to the orifice. In view of this, recently there has been a trend toward providing airless spraying apparatus with tip guards or nozzle guards which fit over the spray tip so as to prevent the human operator from placing his hand or any part of his body in sufficiently close proximity to the spray tip orifice as to create a potential for liquid injection of material emanating from the spray tip orifice.
Airless spray tip guards are typically suitable for mounting to the nozzle end of the barrel or body of an airless spray gun, and are typically axially aligned and outwardly concentric with respect to the axis of the spray tip orifice and the axis of the spray which emanates therefrom.
Such tip guards are typically tube-like and flared or notched to accommodate the fan-shaped spray emanating from the typical spray tip orifice. U.S. Pat. No. 3,963,180 discloses a flared nozzle guard whereas U.S. Pat. No. 4,025,045 discloses a substantially cylindrical guard having axial diametrally opposed notches.
In addition, such tip guards typically include axially extending portions which are approximately 3/4 to 1 inch long. The axially extending portions are intended to prevent fingers, hands, etc., from getting dangerously close to the spray tip orifice.
Unfortunately, in spite of the fact that most if not all manufacturers of airless spraying equipment provide such guards, injection accidents continue to occur. An accident can occur when a spray gun is inadvertently activated absent its tip guard. A tip guard might be removed to provide access to the inner portions of the gun so that they can be cleaned or repaired.
Several prior art nozzle guard systems have been developed to eliminate accidents in cases of loosened or removed nozzle guards. For example, U.S. Pat. No. 4,360,132 discloses a safety control apparatus for an automatic airless spray gun. If the safety guard is removed or loosened the air pressure which operates the hydraulic valve within the spray gun is relieved and insufficient air pressure is available to overcome the return spring of the hydraulic valve. U.S. Pat. Nos. 3,944,141 and 3,913,844 each disclose a tip guard which, if removed, prevents the spray gun from being activated. Thus, these tip guards are only directed to accidents which occur due to the loosening or removal of a tip guard.
Sometimes, however, injection accidents are as a result of the axially extending portion(s) of a guard having been sawn or broken off. The techniques disclosed in U.S. Pat. Nos. 4,360,132, 3,944,141 and 3,913,844 cannot prevent injection accidents when a guard is in place but has been dangerously shortened. The axially extending portion(s) of a tip guard might inadvertantly break off. Alternatively, an operator might intentionally remove the axially extending portion(s) if he thinks that the guard is interfering with the spray or if he wants to have immediate access to the spray tip to clean it.
U.S. Pat. No. 4,181,261, issued to G. Crum, discloses a safety system for an airless spray nozzle which is directed to the problem of a tip guard which has been dangerously shortened, whether accidentally or intentionally. As disclosed in Crum, the system preferably includes a compressed air supply; a tip guard having axially extending portions which form a tip guard cavity; a normally closed pneumatically actuated hydraulic valve; and conduits interconnecting the air supply to the cavity within the tip guard and connecting the tip guard cavity to the pneumatically controlled hydraulic valve. If the air pressure supplied to the normally closed valve is relieved for any reason the valve will close to prevent liquid from flowing to the nozzle portion of the "airless" spray gun. The air pressure would be relieved if the tip guard were removed or if one or both of the axially extending portion(s) were broken or sawn off. If the tip guard cavity is exposed, the air bleeds through the cavity and into the atmosphere and the normally closed hydraulic valve closes to prevent the flow of liquid, e.g., paint.
The Crum technique discussed above is indeed directed to the problem of a broken or truncated tip guard. However, the Crum apparatus requires the use of an auxiliary power supply such as an air supply, and also requires the use of an additional hydraulic valve in the liquid handling system. Thus, the Crum system is not usable if an auxilliary power supply, such as a compressed air supply, is unavailable. Further, the Crum technique is expensive since it requires the additional valve and the conduits which serve to interconnect the pneumatic components. In addition, the air conduits only serve to make the spray gun heavier and less maneuverable.
Importantly, the preferred embodiment of the Crum system shown in the figures can easily be defeated by connecting the air supply directly to the normally closed pneumatically actuated hydraulic valve. If this direct connection is made, the tip guard can be shortened without interfering with the flow of liquid through the spray tip orifice.
The present invention includes a method and apparatus for guarding the spray tip orifice of an airless spraying apparatus. The invention provides a safety technique for discouraging the shortening of a spray tip guard. The technique is very cost effective and does not require additional components such as valves or auxiliary power supplies. Further, the method and apparatus of the present invention are very difficult to override and should effectively prevent the intentional or unintentional shortening of the tip guard.